Polymer-based light-emitting and photovoltaic devices, including those incorporating nanocrystal-containing polymers are known in the art. The performance of polymer-based photovoltaic devices has been improved, e.g., by embedding semiconductor nanocrystals into the polymer matrix. For example, nanocomposite-based photovoltaic devices are described in U.S. Ser. No. 60/408,722, filed Sep. 5, 2002; U.S. Ser. No. 60/421,353, filed Oct. 25, 2002 and U.S. Ser. No. 60/452,038, filed Mar. 4, 2003. However, the performance of these and other devices that employ nanocrystals can be further improved.
Semiconducting nanocrystals can be designed with specific optical properties and/or electronic structures, in part by controlling the size and shape of the nanocrystals used as part of the light harvesting element in the photovoltaic devices. In addition, the polymeric matrix encompassing the nanocrystal can be selected to also absorb light. However, charge transport within the photovoltaic device is generally limited by matrix constraints, rather than by the absorption properties of the nanocrystals. As a result, charge transport through the matrix and/or among the nanocrystals is an important element of optimal photovoltaic operation.
Nanocrystal syntheses typically produce particles having surfaces coated with a surfactant layer, e.g., a layer of molecules having long aliphatic chains, such as alkyl phosphonic acids, alkyl amines, alkyl carboxylic acids, alkyl phosphines or alkyl phosphine oxides. These ligands form a substantially non-conductive layer on the nanocrystal surface. In applications in which it is desirable to efficiently remove or add charges to nanocrystalline structures, the residual aliphatic ligand layer limits the charge transfer to the surface. Synthesis of water soluble semiconductor nanocrystals capable of light emission are described, for example, in U.S. Pat. No. 6,251,303 to Bawendi et al. entitled “Water-soluble fluorescent nanocrystals” (Jun. 6, 2001) and U.S. Pat. No. 6,319,426 to Bawendi et al. titled “Water-soluble fluorescent semiconductor” (Nov. 20, 2001).
While conductive polymers are known in the art (see, for example, H. S. Nalwa (ed.), Handbook of Organic Conductive Molecules and Polymers, John Wiley & Sons 1997; U.S. Pat. No. 6,399,224 to Li, “Conjugated Polymers with Tunable Charge Injection Ability”; and U.S. Pat. No. 5,504,323 to Heeger et al., “Dual function conducting polymer diodes”), these polymers do not have any functional group(s) that can bind strongly to a nanocrystal surface. As such, these polymers do not make optimal contact with the nanocrystal.
The performance of nanocrystal-based light-emitting and photovoltaic devices would be improved if the removal or addition of charges via the nanocrystals was more energetically efficient. Accordingly, there exists a need in the art for improved ligands for use with nanocrystal structures. The present invention meets these and other needs by providing novel compositions for use with nanocrystals, as well as methods for making and using the novel compositions. A complete understanding of the invention will be obtained upon review of the following.